Each year on Yom Kippur, Jews wish each other a khatima tova (a good seal in the Book of Life) and tolerable fast. The route to a khatima tova is beyond the scope of this article; the route to an easy fast is simpler to describe. The following are the essentials of human physiology that will help you have a tolerable fast on Yom Kippur:

Don't get thirsty:

Most people think the
difficulty about fasting is feeling "hungry". However, avoiding thirst is
much more important for how you feel. Not only do you avoid the discomfort of
thirst but you are also well hydrated and swallow frequently, so your stomach
does not feel as empty.

One important way to remain well hydrated is to avoid drinks or foods that cause your body to get rid of water. Such foods and drinks include alcohol, tea, caffeinated coffee and chocolate. Another important rule is to avoid consuming much salt. Salt causes a person to feel thirsty despite having a "normal" amount of water, because extra water is needed for the extra salt. For this reason you should avoid processed foods containing lots of salt such as
pickles, cold cuts, or cheese. Most tomato sauces, canned fish and
smoked fish have a lot of added salt. Since Kosher meat has a high salt content
it may be best to choose a main course such as fresh fish, canned no-salt tuna
fish or a de-salted meat such as boiled chicken.

By avoiding these types of foods and drinks in the several hours before a fast, you can avoid either losing water or needing extra water. Other actions that cause the body to lose water, such as perspiring in warm clothing, should also be avoided during the fast.

Don't start the pre-fast meal on a full stomach:

The pre-fast meal often begins at 5 PM, so a large lunch could prevent you from eating enough immediately before the fast. It is best to have a small lunch, or no lunch at all. A large breakfast early in the day based on cereals, breads and fruits can provide the energy you need during the day, yet these high-fiber foods will be far downstream by the time of the pre-fast meal and will not keep you from eating enough food at the pre-fast meal. A large breakfast is also helpful because it stretches the stomach. After eating breakfast, it is best to consume beverages during the day. This will not fill you up, since liquids are absorbed quickly, and this will ensure that you have absorbed enough fluids during the day to start the pre-fast meal being well hydrated. Be sure to avoid beverages with alcohol or caffeine. You should also drink at least a glass or two of fluids with the pre-fast meal because many foods need extra water to be digested properly.

Eat foods that are digested slowly:

Include some foods high in oils and fats in the pre-fast meal, since such foods delay emptying of the stomach and effectively prolong your meal. However, beware of fatty meats or salted potato chips that could load you up with too much salt. Salads and other high fiber foods that are so important in one's normal diet should be de-emphasized for the pre-fast meal since they travel quickly through the digestive system. Fruit, despite its high fiber content, is worthwhile since it carries a lot of water in a "time-release" form.

Don't get a headache:

Withdrawing from caffeine produces a headache in people who drink several cups of coffee a day. If you consume this much caffeine in coffee or other
foods or drinks you should prepare yourself for the caffeine-free period by reducing or eliminating caffeine from your diet in the days or weeks before Yom Kippur. Don't try to get through the fast by drinking coffee right before Kol Nidre, since this will cause you to lose a lot of water.

Make the meal tasty enough so people will eat:

The pre-fast meal doesn't have be bland. Spices such as lemon or herbs are fine for fasting, but salt and monosodium glutamate should be reduced as much as possible.

Don't do a complete fast if you have certain medical problems:

People with medical conditions
such as diabetes should consult their doctors and rabbis before fasting. Certain medications need to be taken during Yom Kippur, and it is important to swallow them with enough water to avoid pills getting stuck on the way to the stomach and damaging the esophagus. Fasting by women who are pregnant or breast feeding can also be dangerous. If a young person who has not fasted much before has unusual difficulty
fasting you should discuss this with your doctor since this happens in some
serious metabolic problems
in which fasting can be very dangerous.

Don't eat improperly after Neila:

Even people who have prepared well for fasting will be hungry after Neila. Be sure not to eat food too quickly at the post-fast meal. Begin the break-fast meal with several glasses of milk or juice: these put sugar into the bloodstream and occupy space in the stomach, discouraging you from eating too rapidly. Also be careful about eating high salt foods such as lox, since you will still be a little dehydrated and will need to drink a lot of fluids to avoid waking up extremely thirsty in the early morning hours. If you take vitamin C, be sure that you are fully re-hydrated beforehand, because one of
the vitamin's breakdown products can precipitate out in your urine if you are dehydrated, causing kidney stones, which are painful and dangerous.

These preparations for the fast of Yom Kippur will be different from your normal routine, but they can serve as a concrete reminder of the approaching Day of Atonement.